Bacillus thuringiensis (B.t.) is a soil-borne bacterium that produces pesticidal crystal proteins known as delta endotoxins or Cry proteins. Cry proteins are oral intoxicants that function by acting on midgut cells of susceptible insects. Some Cry toxins have been shown to have activity against nematodes. An extensive list of delta endotoxins is maintained and regularly updated at the Bacillus thuringiensis Toxin Nomenclature web site maintained by Neil Crickmore. (See Crickmore et al. 1998, page 808).
Coleopterans are a significant group of agricultural pests that cause extensive damage to crops each year. Examples of coleopteran pests include Colorado potato beetle (CPB), corn rootworm, alfalfa weevil, boll weevil, and Japanese beetle. The Colorado potato beetle is an economically important pest that feeds on the leaves of potato, eggplant, tomato, pepper, tobacco, and other plants in the nightshade family. The Colorado potato beetle is a problematic defoliator of potatoes, in part, because it has developed resistance to many classes of insecticides. Cry toxins, including members of the Cry3, Cry7, and Cry8 family members have insecticidal activity against coleopteran insects.
Although production of the currently-deployed Cry proteins in transgenic plants can provide robust protection against the aforementioned pests, thereby protecting grain yield, adult pests have emerged in artificial infestation trials, indicating less than complete larval insect control. Additionally, development of resistant insect populations threatens the long-term durability of Cry proteins in insect pest control. Lepidopteran insects resistant to Cry proteins have developed in the field for Plutella xylostella (Tabashnik, 1994), Trichoplusia ni (Janmaat and Myers, 2003, 2005), and Helicoverpa zea (Tabashnik et al., 2008). Coleopteran insects likewise have developed resistance in the field to Cry proteins (Gassman et al. PLoS ONE July 2011|Volume 6|Issue 7|e22629). Insect resistance to B.t. Cry proteins can develop through several mechanisms (Heckel et al., 2007; Pigott and Ellar, 2007). Multiple receptor protein classes for Cry proteins have been identified within insects, and multiple examples exist within each receptor class. Resistance to a particular Cry protein may develop, for example, by means of a mutation within the toxin-binding portion of a cadherin domain of a receptor protein. A further means of resistance may be mediated through a protoxin-processing protease.
There is interest in the development of new Cry proteins that provide additional tools for management of coleopteran insect pests. Cry proteins with different modes of action as well as additional Cry transgenic plants can prevent the development of insect resistance and protect the long term utility of B.t. technology for insect pest control.